Distributed Bragg reflectors, or DBRs, are in common use in both passive and active optical elements. A typical application is to form the optical cavity for a vertical cavity surface emitting laser, or VCSEL. A prior art VCSEL is shown in FIG. 1 in a schematic cross-sectional view. In this example, contact 100 makes electrical contact to an n-type DBR 110, and ring contact 101 makes electrical contact to a p-type DBR reflector 130. The region between the two DBRs is the optical cavity 120 of the VCSEL, which comprises a laser medium 123 located between optical spacers 121 and 122. Laser medium 123 typically comprises a semiconductor heterostructure, which can take the form of a set of quantum wells.
The n-type DBR 110 consists essentially of an alternating stack of high-refractive index layers 111 and low-refractive index layers 112. Layers 111 and 112 each typically have an optical thickness nominally equal to a quarter wavelength of the light which the VCSEL is intended to emit. Thus, layers 111 have a smaller physical thickness than do layers 112, as shown in FIG. 1. Similarly, the p-type DBR 130 consists essentially of an alternating stack of high-refractive index layers 131 and low-refractive index layers 132. Layers 131 and 132 also typically each have an optical length nominally equal to a quarter wavelength of the intended light to be emitted by the VCSEL.
In operation, a current source (not shown) is attached to contact 100 and ring contact 101, and an electrical current flows vertically through the stack of layers comprising the VCSEL, producing electrons and holes that recombine in the laser medium 123 to produce optical gain for lasing. The small size of the optical cavity 120 is beneficial in that it has a small volume and a small active length. The small volume serves to lower the lasing threshold current relative to that required in competing devices, and also allows faster switching of the laser output. The small cavity length makes it easier to excite only a single laser mode, a property beneficial for many optical communication applications.
The use of such a small optical cavity, however, forces adoption of stringent performance criteria for the distributed Bragg reflectors. The distributed Bragg reflectors must have very high reflectivity at the intended emission wavelength, owing to the small optical gain which can be derived from such a short optical cavity. For the same reason, the distributed Bragg reflectors must exhibit very low absorption of light at the intended emission wavelength. Finally, for purposes of electrical and thermal performance, the distributed Bragg reflectors must have acceptably large electrical and thermal conductivity.
The reflectivity of a distributed Bragg reflector increases both with the number of layers in the reflector, and with the difference in the indices of refraction in the alternating layers. However, large difference in the indices of refraction generally correlates with a large difference in the electronic bandgap, which yields a DBR with smaller electrical conductivity. The design and performance of the distributed Bragg reflectors is generally a limiting factor in the functionality of an intended VCSEL device.
VCSELs which emit in the 1.55 μm (micrometer) spectral range are of particular interest for optical communication applications. Optical fibers exhibit particularly low signal loss in this spectral range. Unfortunately, VCSELs designed for this spectral range have until recently required the fusion of separately grown structures to avoid problems with excess lattice stress.
There are, however, material systems in which a monolithic VCSEL structure can be designed and implemented for the 1.55 μm spectral region without unmanageable mismatch stress. One of the best studied at this point comprises the use of the InP-AlGaAsSb system to form the distributed Bragg reflectors, and in which the laser medium comprises AlGaInAs or InGaAsP quantum wells or equivalent structures.
In the InP-AlGaAsSb system, the AlGaAsSb alloy can be chosen to have a composition near Al0.1Ga0.9As0.52Sb0.48, a composition which substantially lattice matches with InP. The InP has a refractive index of about 3.18, while the AlGaAsSb alloy has a refractive index of about 3.61, meaning that the index contrast between the two materials is about 13%. A distributed Bragg reflector containing 20 pairs of layers of these materials has an optical reflectivity of about 0.994, a value quite suitable for use in VCSELs. These materials also exhibit good electronic and thermal properties for application in VCSELs.
Unfortunately, this material system has the disadvantage of exhibiting significant optical absorptivity in the spectral range of interest. This is the result of spatially indirect photon-assisted electronic transitions from the valence band of the AlGaAsSb alloy to the conduction band of the InP material.
The electronic bandstructure of an abrupt interface 230 between an InP layer 210 and an AlGaAsSb layer 220 is shown schematically in FIG. 2. Here vertical displacement corresponds to energy differences, and horizontal displacement corresponds to physical distance normal to the InP/AlGaAsSb interface 230. The electronic bandgap, or the difference between the conduction band energy and the valence band energy, is roughly 1.345 eV in the InP, and about 0.91 eV in the AlGaAsSb alloy. Both these bandgaps are larger than the intended photon energy, which at 1.55 micron wavelength is about 0.82 eV. As a result, laser photons are not lost in exciting valence electrons up to their corresponding conduction band.
However, what has not been appreciated heretofore is that it is possible for a spatially indirect electronic excitation to be caused by a 0.82 eV photon. Such an event is shown as transition 200 in FIG. 2. Here an electron is excited from an occupied initial state 201 in the valence band in the AlGaAsSb alloy into an empty final state 202 in the conduction band of the InP material. Transition 200 requires a minimum photon energy of about 0.60 eV in this material system. As a result, spatially indirect electronic excitations can result in significant absorption of the desired 1.55 μm (0.82 eV) photons, and hence can degrade or preclude 1.55 μm VCSEL operation.
The influence of spatially indirect photon-assisted electronic transitions is not limited to the InP/AlGaAsSb material system. Any material system in which the difference in energy between the valence band of one material and the conduction band of an adjoining material is less than the photon energies of interest to the intended function of the device allows such transitions, and hence will exhibit optical absorption of the device photons.
There is a need in the art for a VCSEL structure which can be grown monolithically, comprising distributed Bragg reflectors having suitable reflectivity and electrical and thermal transport properties, in which spatially indirect electronic transitions are inhibited. Other uses of such low-absorption distributed Bragg reflectors will be clear to one skilled in the art.